feat(server): increase embedding jobs concurrency & handle empty content after trim (#12574)

<!-- This is an auto-generated comment: release notes by coderabbit.ai -->
## Summary by CodeRabbit

- **Improvements**
  - Increased the default concurrency for background tasks, enhancing processing efficiency.
  - Improved handling of empty or unsupported documents to ensure consistent processing.
  - Optimized document filtering to exclude certain documents from processing, improving performance.

- **Bug Fixes**
  - Enhanced detection of empty document summaries, reducing errors during processing.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

packages/backend/server/src/base/job/queue/config.ts

@@ -50,7 +50,7 @@ defineModuleConfig('job', {
   'queues.copilot': {
     desc: 'The config for copilot job queue',
     default: {
-      concurrency: 5,
+      concurrency: 10,
     },
     schema,
   },